1. Field of the Invention
This invention relates to a linear motor suitable for moving an optical head for the irradiation of a beam of laser light in an optical disk recording/reproducing apparatus.
2. Description of the Prior Art
An optical head for irradiating a beam of laser light upon a record face of a disk in a photo-electromagnetic disk recording/reproducing apparatus or the like has been put into practical use generally in two different types including a type which is driven by a dc motor and another type which is driven by a linear motor. An optical head of the type which is driven by a linear motor is advantageous in that the driving mechanism therefor is comparatively simple and the velocity control can be achieved comparatively readily.
In such arrangement, the velocity of movement of the optical head carried on the linear motor is controlled such that the back electromotive force generated in a driving coil of the linear motor is detected and a drive voltage is supplied in time division to the driving coil, but when the drive voltage is not supplied, the velocity of movement of the linear motor is detected from the back electromotive force constant of the driving coil to control the movement of the optical head.
Such a linear motor includes a single coil which serves as a drive coil and also as a detecting coil and is simple in winding thereof. However, it has a drawback in that, since the back electromotive force constant is low, a velocity detection signal having a high S/N ratio cannot be obtained therefrom. Accordingly, it has a problem that the optical head does not perform an accurate seeking operation.
Also an alternative arrangement has been proposed wherein an optical head is moved by a pair of thread mechanisms so that it may be moved by a linear motor formed from one of the thread mechanisms while a velocity thereof is detected mainly by another linear motor formed from the other thread mechanism. Such arrangement, however, has a problem that, since the linear motors are not good in balance, a rattling movement of the optical head takes place upon sudden movement thereof, which results in an increase in the seeking time.